“I confirmed one fatality. A woman. She was under the wheel of a bus. She died, confirmed her death at the scene.”

“There were people across the bridge. There were some with minor injuries, some catastrophic. Some had injuries they could walk away from or who have life-changing injuries.

“There were maybe a dozen (injured).”

Mark Rowley, assistant commissioner of the Metropolitan Police, said the police are “satisfied at this stage it looks like there was only one attacker.”

London Ambulance Service says it has treated at least 10 patients on Westminster Bridge.

An eyewitness has told Sky News he saw a body “face down” in the Thames. Witnesses said people either jumped or were knocked over the side of the bridge as the car mounted the pavement and struck a number of pedestrians.

A spokesman for the Port of London Authority, which looks after safety on the river, said a female member of the public had been recovered from the water near Westminster bridge after she “fell” or “jumped” in.

“A female member of the public was recovered alive from the water, but with serious injuries. She has been brought ashore and is undergoing urgent medical treatment. The working assumption is that she fell or jumped from the bridge.”

Shocking videos and photographs show people lying on the bridge, bleeding heavily, with members of the public rushing to their aid. There is no official estimate of the number injured in the incident.

“I came out of Westminster Tube at about 2:45pm when police were locking down the tube, ushering people out away from Parliament. I was ushered onto Westminster Bridge, and then I looked across the bridge, onto the west pavement,” he recalled.

“For the whole length of the bridge, from south to north, there were people lying on the ground and I was then told that a car had driven from the south end all the way up the pavement to the north end of the bridge. I counted at least eight people clearly injured on the ground.”

Another witness described seeing a middle-aged man carrying a knife running towards the Palace of Westminster.

Jayne Wilkinson told Sky News: “We were taking photos of Big Ben and we saw all the people running towards us, and then there was an Asian guy in about his 40s carrying a knife about seven or eight inches long.

“And then there were three shots fired, and then we crossed the road and looked over. The man was on the floor with blood.

“He had a lightweight jacket on, dark trousers and a shirt.

“He was running through those gates, towards parliament, and the police were chasing him.”

According to Doug Weeks, terrorism expert at London Metropolitan University, the attack, while softened by the high security presence in the area, could have been far bloodier.

“Well certainly within the Parliament compound itself, that is by far one of the most secure areas within London. There are numerous police officers and other security personnel that work in that area as well as around that area, thus it’s not surprising that the police have a very rapid response to this incident,” Weeks told RT.

“Given the location and the number of people at Westminster bridge at any particular time – that is a very crowded place – and reports so far are saying that there is likely five or six people that are injured. And although I’m not saying by any measure that this incident is a minor incident, but given the number of people that frequent that location, I would also have to say this could have been significantly worse,” he said.

Number 10 says prime minister Theresa May is safe. The spokesman declined to say where May was when the attack took place.

It is understood, however, she was rushed to a car 40 yards from the gates outside parliament where the shots were fired minutes after the attack. She was ushered by at least eight armed undercover police, some with their firearms drawn.

The leader of the House of Commons David Lidington says an alleged assailant was shot by armed police “inside the parliament perimeter.”

“Colleagues will have appreciated that events have been moving rapidly and I want to emphasise that the knowledge that I have which is definite is so far very limited,” Lidington told MPs.

“What I am able to say to the House is that there has been a serious incident within the estate. It seems that a police officer has been stabbed; that the alleged assailant was shot by armed police. An air ambulance is attending the scene to remove the casualties. There are also reports of further violent incidents in the vicinity.”

Parliament and the surrounding area is in lockdown. Staff inside parliament have been told to stay inside their offices.

The London Eye has stopped and is currently holding all its “guests” within the attraction.

An air ambulance landed in Parliament Square shortly after the incident.

London Ambulance Service deputy director of operations Pauline Cranmer said it has “declared a major incident and our priority is to assess patients and ensure that they are treated and taken to hospital as soon as possible.”

The US State Department says it is closely following the “concerning situation” in the UK.

“The Department of Homeland Security stands in solidarity alongside our friends in the United Kingdom in condemning the terrible attack at Parliament today. With our partners in federal law enforcement, we are in close contact with our British counterparts to monitor the tragic events and to support the ongoing investigation,” it said in a statement.

The Muslim Council of Britain has issued a statement on the attack, which has not yet been claimed by any group.